Absorbent articles for personal hygiene such as disposable infant diapers, feminine protection pads and adult incontinence undergarments, are designed to absorb and contain body exudates such as urine. These absorbent articles usually comprise several layers having different functions, for example a topsheet, a backsheet and in-between an absorbent core, among other layers. The absorbent core's function is to absorb and retain the exudates for a prolonged amount of time, for example overnight for a diaper, minimize re-wet to keep the wearer dry and avoid soiling of clothes or bed sheets.
The majority of currently marketed absorbent cores for diapers comprise as absorbent material a blend of comminuted wood pulp made of cellulose fibers (“Airfelt”) with superabsorbent polymer particles (herein “SAP”), also called absorbent gelling materials (AGM). Typically the SAP are distributed in these cores so that they are more concentrated in the area where more fluid acquisition capacity is needed, in particular in the crotch (middle) region.
Absorbent cores can expand to several times their initial volumes when saturated and their performance can be impacted by the ability to maintain structural integrity, both in dry and wet conditions. At relatively high concentrations, the SAP tend to separate from the cellulose fibers and to move freely in the absorbent structure. It is thus desirable that the cores in the wet, expanded state, maintain their structural integrity and do not break or burst even when subjected to a shock such as a child sitting heavily on his diaper.
With this type of airfelt core, an increase of capacity to cover higher user loadings will also necessarily imply an increase of pulp or cellulose fiber amount and caliper, which impacts cost and fit. Furthermore, although it is desirable to increase the production speed of these absorbent cores, it becomes more and more difficult to accurately place the SAP according to the desired distribution as the production speed increases. The lack of precision of the distribution of SAP can be compensated by adding more of SAP, which is inefficient and expensive. It is desirable that absorbent cores should be thin (at least when dry) and require as little material as possible for cost and environmental reasons.
Over the last decades continued efforts have been made to develop new absorbent cores addressing these needs, as can be seen from the abundant patent literature. Absorbent articles having a core composed essentially of SAP as absorbent material (so-called “airfelt free” cores) have been proposed. For example US2008/0312623 (Hundorf) describes an airfelt free core having absorbent particulate polymer material with a saline flow conductivity greater than about 100×10−7 cm3·sec/g such that even in the swollen state, i.e., when liquid has been absorbed, the liquid flow throughout the material is not substantially obstructed. However an increased permeability of the SAP is often gained at a cost of reduced absorbent capacity of such SAP. T his may lead to increased usage of SAP in the absorbent structure to match the intended total product capacity. It can therefore be desirable to reduce or eliminate the utilization of such high permeable SAP. Additionally in general such airfelt free absorbent cores lack the flexibility for manufacturers to react to pulp/SAP price changes by accordingly adjusting the absorbent material formulation to match desired performance at the minimum cost.
U.S. Pat. No. 5,593,399 (Tanzer et al.) discloses disposable garments which include discrete pockets of superabsorbent polymer material held between a pair of carrier sheets to provide an absorbent laminate. The garment can comprise an absorbent structure which includes a retention portion having a primary absorbent portion, such as the absorbent laminate, for storing and holding absorbed liquids, such as urine. The retention portion can also include a supplemental absorbent, such as an outerside distribution layer, and alternatively or additionally include a bodyside distribution layer.
U.S. Pat. No. 5,830,202 (Bogdanski et al.) discloses an absorbent structure having a first layer comprising a mixture of AGM and cellulose and/or synthetic fibers, and a second layer comprising liquid-permeable substrate and AGM attached to the substrate thus forming a laminate. The laminate can be located on top of the mixed layer and defines an acquisition zone of low basis weight of AGM particles. The laminate may also be located below the mixed layer. The AGM may be attached to the substrate via a layer of adhesive applied to the substrate or the AGM may be coated by a stream of adhesive prior to contacting the substrate to form adhesively coated particles. Alternatively the AGM particles may be interconnected by the application of an interparticle cross-linking agent to form an interpartically cross-linked aggregate, in this case the AGM particles may be bonded to the substrate by the interparticle crosslinking agent. Finally it is also described the possibility to bond the AGM particles without the usage of an adhesive: the particles can be deposited onto a moist substrate such that the particles absorb moisture on their surfaces and become tacky; with subsequent drying of the moist substrate under application of pressure, resulting in attachment of the particles to the substrate. All of the methods above to attach and bond the AGM to the substrate have limitations, as they are limited to low AGM basis weight or are expensive or are effective only in dry or wet state but not in both conditions or have process issues particularly at high production speed.
Many other absorbent core designs have been proposed in the patent literature, for example U.S. Pat. Nos. 5,562,645 and 6,329,565 (both to Tanzer), U.S. Pat. No. 6,664,437 (Sawyer), U.S. Pat. No. 6,972,011B2 (Maeda), EP1,447,066 (Busam), EP631,768 (Plischke), US2008/0312622 (Hundorf), U.S. Pat. No. 7,938,813 (Wang), U.S. Pat. No. 7,850,672 (Guidotti), WO2009/008788 (Fernkvist), EP1,632,206 (Nakaoka).
Although the currently practiced absorbent cores can provide very good fluid absorbency, fluid retention and fit, it is still desirable to find new and improved cores.
The present inventors have now developed new absorbent cores, which can be produced at increased speed as well as providing design degrees of freedom in terms of profiling and shaping the distribution of SAP in the three spatial directions while keeping material costs as low possible. In particular the cores of the invention can deliver improved dry and wet immobilization of the superabsorbent particles, increased the overall integrity of the absorbent core structure, improve fluid handling performances (including reduced rewet and increased acquisition speed) versus existing airfelt cores.
Furthermore the absorbent cores of the invention containing some amount of cellulose fibers allow increased design flexibility compared to airfelt-free core, can reduce or eliminate the utilization of high permeable SAP and provide flexibility for manufacturers to react to pulp/SAP price changes by accordingly adjusting the absorbent material formulation to match desired performance at the minimum cost.